buildserver: use py3's pathlib to generate cache file URL
buildserver: use py3's pathlib to generate cache file URL

This should handle edge cases better, like odd characters in the path, etc.

fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver